U.S. patent number 4,475,551 [Application Number 06/414,353] was granted by the patent office on 1984-10-09 for arrhythmia detection and defibrillation system and method.
This patent grant is currently assigned to Mieczyslaw Mirowski. Invention is credited to Marlin S. Heilman, Alois A. Langer.
United States Patent |
4,475,551 |
Langer , et al. |
October 9, 1984 |
Arrhythmia detection and defibrillation system and method
Abstract
The invention relates to an arrhythmia detection system and
method for defibrillating the heart of a patient experiencing
abnormal cardiac rhythm, wherein the abnormal cardiac rhythm
(comprising one of fibrillation, high rate tachycardia, and low
rate tachycardia) is first detected, the heart rate is sensed so as
to distinguish between fibrillation and high rate tachycardia, on
the one hand, and low rate tachycardia, on the other hand, and
automatic defibrillation of the heart of the patient is implemented
when one of fibrillation and high rate tachycardia is determined.
In one embodiment, base and apical electrodes are connected to a
probability density function (PDF) circuit and a rate circuit. When
both abnormal cardiac rhythm and excessively high heart rate are
detected, defibrillation of the heart of the patient is
implemented. In a second embodiment, a sensing button is also
connected to the heart, and a switch is interposed between the
electrodes and sensing button, on the one hand, and the interface,
on the other hand. During monitoring of cardiac rhythm by the PDF
circuit, the switch automatically connects the electrodes to the
PDF circuit, while, during sensing of the heart rate, the switch
connects the sensing button to the heart rate circuit. Upon
detection of the need for defibrillation, the switch automatically
connects the defibrillation pulse generator to the electrodes.
Further features include a timed reset capability.
Inventors: |
Langer; Alois A. (Pittsburgh,
PA), Heilman; Marlin S. (Gibsonia, PA) |
Assignee: |
Mieczyslaw Mirowski (Owings
Mills, MD)
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Family
ID: |
26871469 |
Appl.
No.: |
06/414,353 |
Filed: |
September 2, 1982 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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175670 |
Aug 5, 1980 |
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Current U.S.
Class: |
607/5;
600/518 |
Current CPC
Class: |
A61N
1/3956 (20130101); A61N 1/3987 (20130101) |
Current International
Class: |
A61N
1/39 (20060101); A61N 001/36 () |
Field of
Search: |
;126/419D |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Stratbucker et al., "Rocky Mountain Engineering Society", 1965, pp.
57-61..
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Fleit, Jacobson, Cohn &
Price
Parent Case Text
The present application is a continuation of prior U.S. patent
application Ser. No. 175,670, filed Aug. 5, 1980, now abandoned.
Claims
What is claimed is:
1. A method for delivering defibrillating energy to the heart of a
patient experiencing abnormal cardiac rhythm, including ventricular
fibrillation, which is characterized by an irregular ECG waveform,
and high rate tachycardia and low rate tachycardia, both of which
are characterized by regular R-waves occurring at generally stable
rates, the deliverance of defibrillating energy taking place
irrespective of the regularity of the ECG waveform representing
such abnormal cardiac rhythm, said method comprising the steps
of:
monitoring an ECG signal from the heart of the patient;
examining said ECG signal to detect said abnormal cardiac rhythm
including ventricular fibrillation, high rate tachycardia, and low
rate tachycardia;
examining said ECG signal to sense heart rate so as to distinguish
between ventricular fibrillation and high rate tachycardia, on the
one hand, and low rate tachycardia, on the other hand, said
distinguishing being made without regard to the regularity or lack
of regularity of the ECG waveform corresponding to said ECG
signal;
issuing a defibrillating signal when both examining steps sense
ventricular fibrillation or high rate tachycardia;
not issuing a defibrillating signal when both examining steps sense
low rate tachycardia or normal sinus rhythm; and
responding to said defibrillating signal by automatically
delivering defibrillating energy to the heart of the patient.
2. The method of claim 1, wherein said monitoring step comprises
sensing an electrocardiograph (ECG) of the heart to develop ECG
data, and said first recited examining step comprises processing
said ECG data in accordance with a probability density
function.
3. The method of claim 1, comprising the additional step, prior to
said monitoring step, of connecting a base electrode and an apical
electrode to said heart, said base and apical electrodes being used
during said monitoring step.
4. The method of claim 1, comprising the additional step, prior to
said monitoring step, of connecting a base electrode, an apical
electrode and a sensing button to said heart, said base and apical
electrodes being used during said monitoring step, and said sensing
button being used during said monitoring step.
5. The method of claim 1, wherein said two examining steps are
performed simultaneously.
6. The method of claim 5, wherein said monitoring step comprises
sensing an electrocardiograph (ECG) of the heart to develop ECG
data, and said first recited examining step comprises processing
said ECG data in accordance with a probability density function to
produce a detection output when said abnormal cardiac rhythm is
detected, said method comprising the additional step of inhibiting
said detection output when said sensing step determines the
presence of low rate tachycardia.
7. The method of claim 1, wherein said first recited examining step
and said second recited examining step are performed in sequence,
said second recited examining step being performed only when said
abnormal cardiac rhythm is detected during said first examining
step.
8. The method of claim 7, comprising the additional step, prior to
said monitoring step, of connecting a base electrode, an apical
electrode and a sensing button to said heart, said base and apical
electrodes being used during said detecting step, and said sensing
button being used during said sensing step.
9. The method of claim 1, wherein said second recited examining
step comprises comparing the heart rate of the heart to a
predetermined threshold, said step of automatically delivering
defibrillating energy to the heart being executed when said heart
rate exceeds said predetermined threshold.
10. The method of claim 9, wherein said second recited examining
step further comprises automatically returning to said first
recited examining step when said heart rate remains below said
predetermined threshold for a predetermined time.
11. A system for delivering defibrillating energy to the heart of a
patient experiencing abnormal cardiac rhythm including ventricular
fibrillation which is characterized by an irregular ECG waveform,
and high rate tachycardia and low rate tachycardia, both of which
are characterized by regular R-waves occurring at generally stable
rates, the deliverance of defibrillating energy taking place
irrespective of the regularity of the ECG waveform representing
such abnormal cardiac rhythm, said system comprising:
monitoring means for deriving an ECG signal from the heart of the
patient;
detecting means, receiving said ECG signal from said monitoring
means for detecting said abnormal cardiac rhythm, said abnormal
cardiac rhythm including ventricular fibrillation, high rate
tachycardia, and low rate tachycardia, said detecting means
normally issuing a defibrillating signal upon detection of said
abnormal cardiac rhythm;
automatic defibrillating means responsive to said defibrillating
signal for automatically delivering defibrillating energy to the
heart of the patient;
sensing means receiving said ECG signal from said monitoring means
and simultaneously operative with said detecting means for sensing
heart rate so as to distinguish between ventricular fibrillation
and high rate tachycardia, on the one hand, and low rate
tachycardia, on the other hand, said distinguishing being made
without regard to the regularity or lack of regularity of the ECG
waveform corresponding to said ECG signal; and
means responsive to said sensing of said ventricular fibrillation
and high rate tachycardia by said sensing means for permitting said
defibrillating signal to activate said automatic defibrillating
means and responsive to said sensing of lowa rate tachycardia by
said sensing means for preventing deliverance of said
defibrillating signal to said automatic defibrillating means.
12. The system of claim 11, wherein said monitoring means comprises
a sensing circuit for sensing an electrocardiograph (ECG) of the
heart to develop ECG data, and said detecting means comprises
processing means for processing said ECG data in accordance with a
probability density function.
13. The system of claim 11, further comprising a base electrode and
an apical electrode connected to said heart, and means for
connecting said base and apical electrodes to said detecting means
and said sensing means.
14. The system of claim 11, wherein said monitoring means comprises
a sensing circuit for sensing an electrocardiograph (ECG) of the
heart to develop ECG data, and said sensing means comprising a low
pass filter circuit connected to said sensing circuit for low pass
filtering said ECG data to provide a low pass filter output, said
sensing means further comprising a rate circuit connected to said
low pass filter circuit for receiving said low pass filter output
and responsive thereto for sensing the heart rate.
15. The system of claim 11, wherein said detecting means and said
sensing means operate simultaneously, said detecting means
including a processing circuit for processing said ECG signal in
accordance with a probability density function to develop a
detection output, said sensing means comprising a rate circuit
responsive to detection of low rate tachycardia for inhibiting said
detection output of said processing circuit, whereby to inhibit
automatic defibrillation of the heart of the patient upon detection
of said low rate tachycardia.
16. The system of claim 11, wherein said monitoring means comprises
a base electrode, an apical electrode and a sensing button
connected to said heart, and said system further comprises
connecting means for connecting said base and apical electrodes to
said detecting means, and for connecting said sensing button to
said sensing means.
17. The system of claim 16, wherein said connecting means comprises
a switch circuit having a first state and a second state, said
switch circuit being initially in said first state during operation
of said detecting means for detecting said abnormal cardiac rhythm,
said switch circuit being responsive to detection of said abnormal
cardiac rhythm by said detecting means so as to be actuated to said
second state, said switch circuit being responsive to determination
of one of said ventricular fibrillation and said high rate
tachycardia so as to be actuated to said first state for automatic
defibrillation of the heart of the patient by said automatic
defibrillating means.
18. The system of claim 17, wherein said detecting means comprises
a probability density function circuit for determining when said
abnormal cardiac rhythm exists, and a bistable circuit connected to
said probability density function circuit and responsive to
determination of the existence of said abnormal cardiac rhythm by
said probability density function circuit for being actuated to a
first state so as to emit a first output indicating existence of
said abnormal cardiac rhythm.
19. The system of claim 18, further comprising means for providing
said first output of said bistable circuit to said switch circuit,
said switch circuit being responsive thereto for being actuated to
said second state.
20. The system of claim 19, further comprising timed reset circuit
means for automatically resetting said bistable circuit, and means
for providing said first output of said bistable circuit to said
timed reset circuit means for actuating said timed reset circuit
means to start counting to a predetermined final count, said timed
reset circuit means resetting said bistable circuit upon reaching
of said predetermined final count.
21. The system of claim 18, wherein said sensing means comprises a
heart rate circuit responsive to said first output of said bistable
circuit for starting a heart rate monitoring operation.
22. The system of claim 21, wherein said heart rate circuit issues
a first output when said monitored heart rate of the patient
exceeds a predetermined threshold.
23. The system of claim 22, further comprising AND gate means
responsive to said first output of said bistable circuit and said
first output of said heart rate circuit for actuating said
automatic defibrillating means to defibrillate the heart of the
patient.
24. The system of claim 23, wherein said AND gate means is
responsive to said first output of said bistable circuit and said
first output of said heart rate circuit for actuating said switch
circuit to said first state.
25. The system of claim 23, wherein said AND gate means is
responsive to said first output of said bistable circuit and said
first output of said heart rate circuit for automatically resetting
said bistable circuit.
26. The system of claim 18, further comprising timed reset circuit
means responsive to issuance of said first output by said bistable
circuit for starting a counting operation so as to count to a final
count value, said timed; reset circuit means being responsive to
reaching of said final count value for resetting said bistable
circuit.
27. The system of claim 17, wherein said switch circuit, in said
first state, connects said base and apical electrodes to said
detecting means, and, in said second state, connects said sensing
button to said sensing means,
28. The system of claim 17, further comprising interface means for
interfacing said switch circuit to said detecting means and said
sensing means, said switch circuit, in said first state, connecting
said base and apical electrodes to said interface means, and, in
said second state, connecting said sensing button to said interface
means.
29. The system of claim 28, further comprising an ECG amplifier for
connecting said interface means, on the one hand, to said detecting
means and said sensing means, on the other hand.
30. The system of claim 17, further comprising an ECG amplifier for
connecting said switch circuit, on the one hand, to said detecting
means and said sensing means, on the other hand.
31. The system of claim 11, wherein said monitoring means comprises
an ECG input circuit for sensing an ECG of the heart to develop ECG
data, and said detecting means comprises a processing circuit for
processing said ECG data in accordance with a probability density
function, and for issuing a first output when said probability
density function is satisfied, said detecting means further
comprising a bistable circuit responsive to said first output of
said processing circuit for issuing a further output signal
indicating detection of said abnormal cardiac rhythm.
32. The system of claim 31, wherein said monitoring means comprises
a base electrode, an apical electrode, and a sensing button
connected to said heart, and said system further comprises
connecting means for initially connecting said base and apical
electrodes to said ECG input circuit, said connecting means being
responsive to said further output signal from said bistable circuit
for connecting said sensing button to said sensing means.
33. The system of claim 31, wherein said sensing means comprises a
heart rate circuit for monitoring the heart rate of the patient,
said heart rate circuit being responsive to said further output
signal from said bistable circuit for beginning its heart rate
monitoring operation.
34. The system of claim 33, wherein said heart rate circuit issues
an output signal when said heart rate exceeds a predetermined
threshold, said system further comprising AND gate means responsive
to said further output signal from said bistable circuit and said
output signal of said heart rate circuit for issuing an output
signal actuating said automatic defibrillating means to
defibrillate the heart of the patient.
35. The system of claim 34, further comprising means for providing
said output-signal of said AND gate means to said bistable circuit,
said bistable circuit being responsive thereto for resetting itself
so as to stop issuing said further output signal.
36. The system of claim 34, further comprising a base electrode, an
apical electrode, and a sensing button, and connecting means for
initially connecting said base and apical electrodes to said ECG
input circuit, said connecting means being responsive to said
further output signal of said bistable circuit for connecting said
sensing button to said heart rate circuit.
37. The system of claim 36, further comprising means for providing
said output signal of said AND gate means to said connecting means,
said connecting means being responsive to said output signal of
said AND gate means for connecting said base and apical cup
electrodes to said automatic defibrillating means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an arrhythmia detection system and
method, and more particularly to an improved system and method for
defibrillating the heart of a patient when the patient experiences
life-threatening fibrillation.
2. Description of the Prior Art
In recent years, substantial progress has been made in the
development of defibrillation techniques for providing an effective
medical response to various heart disorders or arrhythmias
particularly ventricular fibrillation which is characterized by an
irregular ECG waveform. Earlier efforts resulted in the development
of an electronic standby defibrillator which, in response to
detection of abnormal cardiac rhythm, discharged sufficient energy,
via electrodes connected to the heart, to depolarize the heart and
restore it to normal cardiac rhythm. Examples of such electronic
standby defibrillators are disclosed in commonly assigned U.S. Pat.
No. 3,614,954 (subsequently, U.S. Pat No. Re. 27,652) and U.S. Pat.
No. 3,614,955 (subsequently, U.S. Pat. No. Re. 27,757).
Past efforts in the field have also resulted in the development of
implantable electrodes for use in accomplishing ventricular
defibrillation (as well as other remedial techniques). In
accordance with such techniques, as disclosed (for example) in U.S.
Pat. No. 4,030,509 of Heilman et al, an apex electrode is applied
to the external intrapericardial or extrapericardial surface of the
heart, and acts against a base electrode which can be either
similarly conformal or in the form of an intravascular catheter.
Such electrode arrangements of the prior art, as disclosed in the
aforementioned patent of Heilman et al, can employ independent
pacing tips associated with either a base electrode or an apex
electrode, or both.
Recent efforts also have resulted in the development of techniques
for monitoring heart activity (for the purpose of determining when
defibrillation or cardioversion is necessary), which techniques
employ a probability density function for determining when
ventricular fibrillation is present. Such a technique, employing
the probability density function, is disclosed in U.S. Pat. Nos.
4,184,493 and 4,202,340, both of Langer et al.
In accordance with this latter technique of the prior art, when the
probability density function is satisfied, fibrillation of the
heart is indicated. However, recent experience has shown that, with
one or more particular abnormal ECG patterns, the prior art
probability density function detector, if not optimally adjusted,
can be "triggered" not only by actual ventricular fibrillation, but
also by some forms of high rate ventricular tachycardia, and low
rate ventricular tachycardia as well, particularly in the presence
of ventricular conduction abnormalities. Unlike ventricular
fibrillation, such high rate and low rate tachycardias are
characterized by regular R-waves occurring at generally stable
rates. The possibility of such triggering in the presence of high
rate tachycardia is acceptable because high rate tachycardia can be
fatal if present at such a rate that sufficient blood pumping no
longer is accomplished. However, triggering in the presence of
non-life threatening, low rate tachycardia could be considered a
problem. Therefore, it has been determined that there is a need for
a system and method for distinguishing between ventricular
fibrillation and high rate tachycardia, on the one hand, and low
rate tachycardia, on the other hand.
It is worth noting that prior art implementation of the probability
density function technique was, for a time, limited to "triggering"
only in the presence of ventricular fibrillation. This was
accomplished by adjusting the decision boundaries of the
probability density function detector in a conservative way so as
to "trigger" only upon occurrence of life-threatening ventricular
fibrillation. However, it was soon realized that there existed
situations in which it was desirable to take remedial action upon
the detection of high rate tachycardia, as indicated by occurrence
of a heart rate above a lower threshold level (for example, above
200 beats per minute). This was initially accomplished merely by
adjusting the probability density function criteria so as to be
"triggered" at the lower threshold level.
However, it was soon discovered that a problem existed in a
detector with relaxed decision criteria, in that extraordinary
types of ECG signals were capable of "triggering" the modified
probability density function detector, even though neither
ventricular fibrillation nor high rate tachycardia was present.
Therefore, it has been determined that there is a need for an
arrhythmia detection system and method which not only performs a
probability density function analysis, but which also includes some
technique for distinguishing between fibrillation and high rate
tachycardia, on the one hand, and low rate tachycardia, on the
other hand. Thus, the inventive system and method herein disclosed
amounts to a "backup" technique by means of which high rate
tachycardia is treated by issuance of a defibrillating shock to the
patient, while low rate tachycardia is not so treated.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an arrhythmia
detection system and method, and more particularly an improved
system and method for defibrillating a heart which is undergoing
abnormal cardiac rhythm, the improved system and method employing a
technique for distinguishing between ventricular fibrillation and
high rate tachycardia, on the one hand, and low rate tachycardia,
on the other hand. More specifically, the system and method of the
present invention, besides utilizing the probability density
function technique to determine the presence of abnormal cardia
rhythm, also employs heart rate sensing for the purpose of
distinguishing between ventricular fibrillation and high rate
tachycardia, on the one hand, the latter being indicated by a heart
rate above a predetermined threshold, and low rate tachycardia, on
the other hand, the latter being indicated by a heart rate falling
below the predetermined threshold.
The present invention is implemented by a first preferred
embodiment of a system, wherein a superior vena cava (or base)
electrode and an apical (or patch) electrode are associated with
the heart, and are employed, as in conventional in the art, not
only to derive an electrocardiograph (ECG) signal, but also to
apply a defibrillating shock to the heart. It should be noted that
the ECG amplifier in the first embodiment essentially provides the
derivative of the heart signal as taught in U.S. Pat. No.
4,184,493. However, in contrast to the prior art, the
differentiated ECG signal is, in this first embodiment of the
invention, applied to a probability density function circuit, and
to a low pass filter and a heart rate circuit, by means of which
the probability density function and heart rate, respectively, are
obtained. In accordance with this first embodiment, satisfaction of
the probability density criteria (that is, determination of whether
the time-averaged derivative of the ECG remains off the base line
for extended periods of time) while the heart rate is above a
predetermined threshold results in actuation of a conventional
defibrillating pulse generator to issue a defibrillating shock to
the heart. Thus, the defibrillating shock will be issued to the
heart only upon the occurrence of fibrillation or high rate
tachycardia, as contrasted with non-life threatening low rate
tachycardia.
In accordance with a second embodiment of the present invention, a
sensing button (preferably, associated with the apical or patch
electrode) is connected to the heart for use in deriving the heart
rate. Thus, in this embodiment, the base and apical electrodes are
utilized initially to derive the ECG signal by means of which the
probability density function is examined. If the probability
density function indicates abnormal cardiac rhythm, a switching
operation takes place, whereby the sensing button is utilized to
derive an ECG signal which is further utilized to determine the
heart rate. Since a very small area electrode will result in
signals in which cardiac depolarizations can still be identified,
even during ventricular fibrillation, a conventional R-wave
detector can be used so as to provide an R-wave for heart rate
sensing. Then, if the heart rate is above the predetermined
threshold, a defibrillating shock is issued. Once a shock is
issued, a further switching operation is executed so that the base
and apical electrodes may be utilized in further examining the
probability density function.
In accordance with a further feature of the present invention, this
second embodiment is provided with a timed reset capability,
whereby, once the probability density function indicates abnormal
cardiac rhythm, if a heart rate above the predetermined threshold
is not indicated within a predetermined time, a return switch
operation is automatically executed so as to permit renewed
monitoring of the base and apical electrodes and examination of the
resulting ECG signal vis-a-vis the probability density
function.
Therefore, it is an object of the present invention to provide an
arrhythmia detection system and method, and more particularly an
improved system and method for defibrillating a heart experiencing
abnormal cardiac rhythm.
It is a further object of the present invention to provide a system
and method which is capable of distinguishing between ventricular
fibrillation and high rate tachycardia, on the one hand, and low
rate tachycardia, on the other hand.
It is a further object of the present invention to provide a system
and method which employs a probability density function technique
to determine the existence of abnormal cardiac rhythm, and which
further employs a heart rate sensing technique for the purpose of
distinguishing between ventricular fibrillation and high rate
tachycardia, on the one hand, and low rate tachycardia, on the
other hand.
It is a further object of the present invention to provide a system
and method, wherein base and apical electrodes are employed both
for monitoring the ECG signal vis-a-vis the probability density
function, and for determining whether or not the heart rate is
above or below a predetermined threshold.
It is an additional object of the present invention to provide a
system and method which employs base and apical electrodes for
monitoring the ECG signal in conjunction with examining the ECG
signal vis-a-vis the probability density function, and which
employs a sensing button to derive heart rate information for
distinguishing between ventricular fibrillation and high rate
tachycardia, on the one hand, and low rate tachycardia, on the
other hand.
It is an additional object of the present invention to provide a
system and method, wherein the probability density function is
first examined to determine whether or not abnormal cardiac rhythm
exists, and wherein, if such abnormal cardiac rhythm does exist,
the heart rate of the patient is then examined to distinguish
between ventricular fibrillation and high rate tachycardia, on the
one hand, in which case a defibrillation pulse is issued, and low
rate tachycardia, on the other hand, in which case a defibrillation
pulse is not issued.
It is an additional object of the present invention to provide a
system and method with a timed reset capability, wherein, once the
probability density function indicates abnormal cardiac rhythm, if
a heart rate exceeding a predetermined threshold is not detected
within a predetermined amount of time, the system and method return
to monitoring of the ECG signal vis-a-vis the probability density
function, and no defibrillation pulse is issued.
The above and other objects that will hereinafter appear, and the
nature of the invention, will be more clearly understood by
reference to the following description, the appended claims, and
the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram of a first embodiment of the arrhythmia
detection system of the present invention.
FIG. 2 is a detailed diagram of the heart rate circuit employed for
detecting heart rate in the embodiment of FIG. 1.
FIG. 3A and 3B are a series of waveform diagrams utilized in
describing the operation of the heart rate circuit of FIG. 2.
FIG. 4 is a block diagram of a second embodiment of the system of
the present invention.
FIG. 5 is a detailed diagram of the heart rate circuit employed for
detecting heart rate in the embodiment of FIG. 2.
FIG. 6 is a series of waveform diagrams utilized in describing the
operation of the heart rate circuit of FIG. 5.
DETAILED DESCRIPTION
The arrhythmia detection system and method of the present invention
will now be described in more detail with reference to FIG. 1,
which is a block diagram of a first embodiment of the system.
Referring to FIG. 1, the system of the present invention (generally
indicated by reference numeral 10) is connected to a superior vena
cava (or base) electrode 12 and an apical (or patch) electrode 14,
the latter being disposed in contact with the heart of the patient
as is known in the prior art (see, for example, U.S. Pat. No.
4,030,509 of Heilman et al mentioned above). In the system 10,
electrodes 12 and 14 are connected, via an interface 16, to an ECG
amplifier 18 which has inherent filtering so as to provide an
approximation of the differentiated ECG.
The ECG amplifier 18 is connected both to low pass filter circuit
19 (which itself is connected to heart rate circuit 21) and to
probability density function (PDF) circuit 20. Heart rate circuit
21 is connected via its inhibit output line (INHIBIT) to the PDF
circuit 20, by means of which the output line heart rate circuit 21
is able to inhibit output from the PDF circuit 20. The output of
the PDF circuit 20 is connected to defibrillation pulse generator
26, the latter being connected via the interface 16 to the
electrodes 12 and 14.
In operation, electrodes 12 and 14 are employed, via the interface
16 (a conventional interface, or isolation circuit), for two
purposes: (1) monitoring of heart activity via ECG amplifier 18,
which develops a differentiated ECG signal output provided to PDF
circuit 20 and low pass filter 21, respectively; and (2)
application of a defibrillating shock from defibrillation pulse
generator 26, via the interface 16, to the heart. More
specifically, PDF circuit 20 monitors the probability density
function of the differentiated ECG output signal of ECG amplifier
18, and, in accordance with conventional techniques (as disclosed,
for example, in U.S. Pat. Nos. 4,184,493 and 4,202,340 of Langer et
al), determines when abnormal cardiac rhythm of the heart exists.
At the same time, the low pass filtered ECG signal, provided as the
output of low pass filter 19, is employed by rate circuit 21 to
determine when the heart rate exceeds a predetermined threshold, at
which time rate circuit 21 removes its inhibiting influence on PDF
circuit 20.
Thus, upon determination of abnormal cardiac rhythm by PDF circuit
20, and of a heart rate above the predetermined threshold by rate
circuit 22, the PDF circuit 20 is allowed to enable defibrillation
pulse generator 26, causing the latter to apply a defibrillating
shock, via interface 16, to the heart.
FIG. 2 is a detailed diagram of the heart rate circuit employed for
detecting heart rate in the embodiment of FIG. 1, while FIGS. 3A
and 3B are a series of waveform diagrams utilized in describing the
operation of the heart rate circuit of FIG. 2.
As seen in FIG. 2, the heart rate circuit 21 comprises operational
amplifier OP1 (which is used as a comparator), transistors Q1
through Q4, resistors R3 through R14, capacitors C2 and C3, and
diodes D1 and D2.
In operation, the heart rate circuit 21 of FIG. 2 functions in the
following manner, with reference to the waveforms shown in FIG. 3.
As previously mentioned, the input to ECG amplifier 18 (FIG. 1)
comprises an undifferentiated ECG signal, as provided by the
electrodes 12 and 14. The undifferentiated ECG signal is
represented by the waveform 100 of FIG. 3A, and is illustrated as a
signal having regular R-waves and a generally stable (or uniform)
rate.
As also previously mentioned, the ECG amplifier 18 amplifies and
filters (differentiates) the ECG signal, and the amplified and
differentiated output of the ECG amplifier 18 is shown as waveform
102 in FIG. 3A. Finally, prior to being provided as an input to the
heart rate circuit 21, the amplified and differentiated ECG signal
from amplifier 18 is filtered, in a manner to be discussed in more
detail below, by low pass filter 19 (made up of resistor Rl and
capacitor Cl).
Referring to FIG. 2, the amplified, differentiated and filtered ECG
signal is provided to the negative input of operational amplifier
OP1, the positive input of which receives a reference input REF via
resistor R3. Operational amplifier OP1 is used as a comparator, and
switches between low and high outputs in accordance with the
relationship between the ECG input and the reference REF. More
specifically, it should be noted that zero crossings in a
derivative waveform (such as the output of amplifier 18 of FIG. 1)
correspond to peaks in the original signal (the original ECG
signal). Accordingly, low pass filter 19 of FIG. 1 filters the
differentiated ECG input provided thereto in such a way that the
output of operational amplifier OP1 (FIG. 2), used as a comparator,
switches at the zero crossings in the derivative waveform
corresponding to major peaks in the ECG input signal. The output of
comparator OP1 appears as waveform 104 in FIG. 3A, and illustrates
the switching action just described.
Further referring to FIG. 2, transistor Q1 has its emitter
connected to one of the offset adjustment terminals of operational
amplifier OP1 so as to add hysteresis to the switching threshold of
the amplifier OP1. This hysteresis, in combination with the
characteristics of the low pass filter 19 of FIG. 1, has the effect
of reducing the sensitivity of the heart rate circuit 21 to smaller
peaks in the ECG input signal. As will be now described, the
remainder of heart rate circuit 21 of FIG. 2 acts as a precision
timer which is responsive to the ECG peaks, as detected by and
indicated by switching of the operational amplifier OP1.
Specifically, a programmable uni-junction transistor Q2 is
connected in series with resistor R5, the latter series combination
being connected between the output of amplifier OP1 and the
collector of transistor Q1. In addition, the gate lead of
transistor Q2 is connected, via resistor R4 to the output of
amplifier OP1. In short, programmable uni-junction transistor Q2 is
connected in such a way as to provide a narrow pulse to the base of
a further transistor Q3 having its base connected via resistors R7
and R8 to the uni-junction transistor Q2, as shown. The narrow
pulse thus provided to the base of transistor Q3 corresponds to the
rising edge of the output of amplifier OP1 (waveform 104 of FIG.
3A), and this narrow pulse is indicated by waveform 106 of FIG. 3A.
The operation of programmable uni-junction transistor Q2, which
thus provides the pulse output shown in waveform 106, will be
evident to those of skill in the art with respect to the
utilization of such devices.
The narrow pulse shown in waveform 106 of FIG. 3A is also shown in
FIG. 3B. This narrow pulse is applied to the base of transistor Q3,
and turns on transistor Q3 at a frequency determined by the
frequency of occurrence of the rising edges of the output of
amplifier OP1, that is, in accordance with a frequency related to
heart rate. Thus, if the heart rate is sufficiently high, capacitor
C2 will experience a voltage build-up as shown in waveform 108 of
FIG. 3B. That is to say, capacitor C2 will experience a build-up of
voltage under the influence of power supply V.sub.s (provided via
resistors R9 and RlO), and will then discharge through transistor
Q3 when that transistor is turned on by receipt of a narrow pulse
(waveform 106 of FIG. 3B) at the base of transistor Q3.
Accordingly, since the voltage of C2, for high heart rate, will not
reach the threshold voltage of programmable uni-junction transistor
Q4, transistor Q4 will not conduct, and the gate voltage of
transistor Q4 (that is, the voltage at the junction between
resistors Rll and R12, diode D2 and transistor Q4) will remain high
(see waveform 110 of FIG. 3B). Thus, the output INHIBIT of rate
circuit 21 will remain high, and will not inhibit the PDF circuit
20 of FIG. 1.
Conversely, if the heart rate is low, conduction of transistor Q3
will be relatively less frequent, and capacitor C2 will charge to
the point where transistor Q4 will "fire", thus discharging
capacitor C2 therethrough. This charging and discharging of
capacitor C2, under these conditions, is indicated by waveform 112
of FIG. 3A. When transistor Q4 "fires" in this manner, its gate
lead is pulled low, and remains low until receipt of the next
narrow pulse applied to the base of transistor Q3. Specifically,
the next narrow pulse received at the base of transistor Q3
provides (as indicated in waveform 112 of FIG. 3A) a slight
negative voltage on capacitor C2, and this slight negative voltage
turns off transistor Q4, returning it to its non-conductive state.
Accordingly, the voltage at the point between resistors Rll and
R12, diode D2 and transistor Q4 returns to a positive polarity, as
indicated by waveform 114 of FIG. 3A.
Thus, firing of transistor Q4 results in the occurrence of a
negative-going pulse (waveform 114 of FIG. 3A) at the
aforementioned junction between resistors R11 and R12, diode D2 and
transistor Q4. Such negative-going pulses are utilized to inhibit
operation of the PDF circuit 20 of FIG. 1 (via the control line
INHIBIT). Specifically, these negative-going pulses are utilized to
remove charge from the integrating capacitor in the PDF circuit 20,
as taught in U.S. Pat. No. 4,184,493 of Langer et al.
To summarize, the operation of PDF circuit 20 (FIG. 1) is inhibited
by rate circuit 21 at low heart rate, but no such inhibiting
function takes place at high heart rate. Thus, at high heart rate,
the PDF circuit 20 proceeds with its normal detection operation,
and enables defibrillation pulse generator 26 in accordance
therewith.
Further referring to FIG. 2, it is to be noted that diode D2 is
provided for the purpose of temperature and voltage stabilization
of the time interval of transistor Q4.
Referring back to FIG. 1, as mentioned earlier, interface 16 is a
conventional interface. More specifically, interface 16 protects
the ECG amplifier 18 from the defibrillation pulses issued by
generator 26, while at the same time permitting the monitoring of
heart activity by ECG amplifier 18. Interface 16 is, for example,
disclosed in more detail in copending application U.S. Ser. No.
215,520 of Langer, entitled "Method and Apparatus for Combining
Pacing and Cardioverting Functions in a Single Implanted Device."
Moreover, the PDF circuit 20 is a conventional circuit for
performing the probability density function, and is, for example,
disclosed in more detail in the aforementioned U.S. Pat. Nos.
4,184,493 and 4,202,340 of Langer et al.
FIG. 4 is a block diagram of a second embodiment of the system of
the present invention. Elements common to both FIGS. 1 and 4 have
been identified by identical reference numerals.
Referring to FIG. 4, the system 30 is shown connected to base and
apical electrodes 12 and 14, and to a sensing button 32 (associated
with apical electrode 14). More specifically, the electrodes 12 and
14 and sensing button 32 are connected, via a switch 34 and
interface 16, both to the ECG amplifier 18 and the defibrillation
pulse generator 26. ECG amplifier 18 is connected, as was the case
in FIG. 1, to the PDF circuit 20, but is also connected to R-wave
detector 22 which is of conventional design and provides a pulse
with each R-wave. R-wave detector 22 is subsequently connected to
rate circuit 23, which is shown in detail in FIG. 5 (to be
discussed below). The output of PDF circuit 20 is connected, via
flip-flop 36, to one input of the AND gate 24, the other input of
which is connected to the output of rate circuit 23. The output of
flip-flop 36 is connected to rate circuit 23, the input of a timed
reset circuit 38 (the output of which is connected to the "reset"
input of flip-flop 36), and to switch 34. Moreover, the output of
AND gate 24 is connected not only to defibrillation pulse generator
26, but also to the "reset" input of flip-flop 36 and to switch
34.
In operation, switch 34 is initially in the position indicated by
reference numeral 40. Therefore, in this mode (subsequently
referred to as the "patch" mode), the base and apical electrodes
are utilized by ECG amplifier 18 which monitors heart activity via
interface 16, switch 34, and aforementioned electrodes 12 and 14.
The resulting ECG signal output from amplifier 18 is provided to
PDF circuit 20 (rate circuit 23 is initially in the "off" state).
Detection of abnormal cardiac rhythm by PDF circuit 20 results in
generation of an output, which is applied to the "set" input of
flip-flop 36. When flip-flop 36 is set, existence of abnormal
cardiac rhythm is "memorized", and a Q output is generated.
The Q output of flip-flop 36 is applied as an "enabling input" to
AND gate 24. It is also applied as a START command to both rate
circuit 23 and timed reset circuit 38. Moreover, the Q output of
flip-flop 36 is provided as signal SENSE to the switch 34,
resulting in actuation of switch 34 to the position indicated by
reference numeral 42. This establishes the "sense" mode of
operation, during which heart rate is monitored by rate circuit 23.
More specifically, actuation of switch 34 to the position indicated
by reference numeral 42 connects the interface 16 to the sensing
button 32, so that heart rate can be monitored by rate circuit 23
via R-wave detector 22, switch 34, interface 16, and ECG amplifier
18. Since the ECG amplifier 18 is connected to a very small surface
area electrode, sharp depolarizations will still be delivered to
R-wave detector 22, resulting in a proper signal indication of
heart rate. It will be recalled that rate circuit 23 was started by
the Q output of flip-flop 36, the latter being issued as a result
of detection of abnormal cardiac rhythm (satisfaction of the
probability density function criteria) by PDF circuit 20.
If and when rate circuit 23 detects a heart rate which exceeds a
predetermined threshold, it issues an output to AND gate 24 which,
as enabled by the Q output of flip-flop 36, provides this output as
an enabling input to defibrillation pulse generator 26. Moreover,
AND gate 24 provides this output to the "reset" input of flip-flop
36 (thus, resetting flip-flop 36), and as an input signal PATCH to
switch 34, actuating switch 34 to the position indicated by
reference numeral 40, thus reestablishing the "patch" mode of
operation of the system 30. Finally, defibrillation pulse generator
26, as enabled by AND gate 24, issues a defibrillation pulse, via
interface 16 and switch 34 (in position 40), to the base and apical
electrodes 12 and 14, respectively, so as to defibrillate the heart
of the patient.
As previously mentioned, the Q output of flip-flop 36 starts the
timed reset circuit 38 upon detection of abnormal cardiac rhythm by
the PDF circuit 20. If, after a predetermined period of time, rate
circuit 23 has not detected a heart rate above the predetermined
threshold, timed reset circuit 38 automatically issues a "reset"
input to flip-flop 36, and provides a further input PATCH to the
switch 34, so as to actuate switch 34 to the position indicated by
reference numeral 40, thus reestablishing the "patch" mode of
operation. Accordingly, the system 30 is provided with the
beneficial feature whereby if, within a predetermined time after
detection of abnormal cardiac rhythm by PDF circuit 20, heart rate
is not detected as exceeding the predetermined threshold, the
system 30 is returned to the "patch" mode of operation so as to
permit further monitoring of the ECG signal by the PDF circuit 20.
That is to say, the timed reset circuit 38 removes the enabling
input from AND gate 24, turns off the rate circuit 23, and returns
the switch 34 to the "patch" position (indicated by reference
numeral 40). Then the PDF circuit 20 monitors the base and apical
electrodes 12 and 14, respectively, via switch 34, interface 16 and
ECG amplifier 18, to once again detect existence of any abnormal
cardiac rhythm.
FIG. 5 is a detailed diagram of the heart rate circuit 23 of FIGS.
4, while FIG. 6 is a series of waveform diagrams describing the
operation of the heart rate circuit 23 of FIG. 4. As seen in FIG.
5, rate circuit 23 comprises input resistor 50, NPN transistor 52,
current source 54, capacitor 56, differential amplifier or
comparison circuit 58, peak detector 60, shift register 62 and AND
gate 64.
In operation, ECG signals (generally indicated by reference numeral
70 of FIG. 6 and shown, for each of the two discrete segments, as
having regular R-waves and a generally stable rate) are provided to
R-wave detector 22 (FIG. 4), the latter generating a pulse train
(generally indicated by reference numeral 75 of FIG. 6)
corresponding thereto. Specifically, this pulse train output of the
R-wave detector 22 is provided, via input resistor 50 (FIG. 5), to
the base of NPN transistor 52. Transistor 52 is turned on as a
result of receipt of each individual pulse in pulse train 75, and
is thus turned on in correspondence to detection of individual
R-waves 72, 74. During those periods of time between individual
R-waves 72 (or 74) in FIG. 6, transistor 52 is non-conductive, and
current source 54 builds up a voltage on capacitor 56. This
build-up of voltage on capacitor 56 is generally indicated by
waveform 76 of FIG. 6.
However, upon occurrence of an R-wave 72 or 74, NPN transistor 52
(FIG. 5) becomes conductive, and capacitor 56 discharges
therethrough (see individual waveforms 78 and 80 of FIG. 6). Thus,
it can be seen that, for a normal heart rate (as indicated by
waveforms 72 of FIG. 6), capacitor 56 discharges at a relatively
low frequency of discharge; thus, current source 54 is able to
build the voltage across capacitor 56 to a relatively high level,
exceeding a predetermined reference REF (indicated by reference
numeral 86 in FIG. 6). Conversely, occurrence of R-waves at an
abnormally high rate (as indicated by waveforms 74 of FIG. 6)
results in discharge of capacitor 56 at a more frequent rate (as
indicated by waveforms 80 of FIG. 6), and the reference REF is not
exceeded.
Further referring to FIG. 5, it is seen that differential amplifier
58 is provided, at its negative input, with a voltage corresponding
to the voltage built up on capacitor 56, and, at its positive
input, with a voltage REF corresponding to the predetermined
reference level 86 of FIG. 6. Thus, referring to FIGS. 5 and 6,
whenever the voltage on capacitor 56 exceeds the predetermined
reference 86 (as in waveforms 78), differential amplifier 58 issues
an output X equal to 0, as indicated by inverted square waves 84 of
FIG. 6, to the shift register 62. Conversely, during those periods
of time when the voltage across capacitor 56 does not exceed the
reference 86 (as in waveforms 80 of FIG. 6), differential amplifier
58 issues an output X equal to 1 to the shift register 62.
Rate circuit 23 of FIG. 5 is further provided with a peak detector
60, which is a conventional circuit for detecting the existence of
peaks in the R-waves 70 of FIG. 6. Upon detection of each peak,
detector 60 issues an input SHIFT to shift register 62. As a
result, the output X, at that time, of differential amplifier 58 is
shifted into an end stage of shift register 62, the contents of
register 62 being accordingly shifted by one place to the
right.
Furthermore, the output of shift register 62 (corresponding to the
contents of each bit or stage thereof) is provided to AND gate 64.
Only detection of all 1's in shift register 62 will result in an
output from AND gate 64. This output of AND gate 64 is provided to
one input of AND gate 24 (FIG. 4), the other input of which
receives the Q output of flip-flop 36, indicating satisfaction of
the probability density function criteria, as determined by PDF
circuit 20. As a result, with both the probability density function
criteria having been satisfied and an excessive heart rate having
been detected, AND gate 24 enables defibrillation pulse generator
26, so as to issue a defibrillation pulse to the heart of the
patient. Conversely, so long as any of the bits of shift register
62 are 0 in value, AND gate 64 does not issue an output, indicating
that a consistently high heart rate has not been detected by rate
circuit 23. Thus, shift register 62 provides a means of remembering
the rates of previous beats. (It should be evident that the greater
the number of bits in the shift register, the higher is the number
of R-waves which must exceed a given rate before a high rate is
indicated.) Thus, defibrillation does not take place, even if the
probability density function is satisfied.
While preferred forms and arrangements have been shown in
illustrating the invention, it is to be clearly understood that
various changes in detail and arrangement may be made without
departing from the spirit and scope of this disclosure.
* * * * *